Spinal cord injury (SCI) patients are known to suffer from autonomic failure as a result of their injury. The magnitude of the dysautonomia resulting from such an injury is difficult to predict or characterize and, in varying degree, it impedes the recovery of physiological homeostasis. This study is intended to investigate the effectiveness of heart rate variability (HRV) analysis as a method of quantifying and characterizing autonomic function in patients with traumatic spinal myelopathy. HRV analysis was carried out in 13 male SCI patients (six tetraplegic, seven paraplegic) and 13 age-matched, able-bodied controls. Twenty-four hour ambulatory and sleep ECG tracings were obtained. Time domain, amplitude, and power spectral analyses were used to study HRV and autonomic function. Both tetraplegic (20+/-12 ms, mean+/-SD) and paraplegic (22+/-8 ms) subjects demonstrated significant loss of low frequency 24-hour HRV compared to able-bodied controls (36+/-14 ms, p < 0.05) and during sleep. This was interpreted as being consistent with predominantly sympathetic denervation uninfluenced by degree of physical activity. There were no significant differences between groups in parasympathetically mediated high frequency HRV. We conclude that HRV analysis is capable of distinguishing between SCI or able-bodied humans and among tetraplegic and paraplegic patients. Patterns of altered HRV may be useful in more completely characterizing or stratifying changes in physiology associated with injury level and may have diagnostic, prognostic, or therapeutic significance.